Several lines of direct evidence now suggest that angiogenesis is essential for the growth and persistence of solid tumors and their metastases (Folkman, 1989; Hori et al., 1991; Kim et al., 1993; Millauer et al., 1994). To stimulate angiogenesis, tumors upregulate their production of a variety of angiogenic factors, including the fibroblast growth factors (FGF and BFGF) (Kandel et al., 1991) and vascular endothelial cell growth factor/vascular permeability factor (VEGF/VPF). However, many malignant tumors also generate inhibitors of angiogenesis, including angiostatin and thrombospondin (Chen et al., 1995; Good et al., 1990; O'Reilly et al., 1994). It is postulated that the angiogenic phenotype is the result of a net balance between these positive and negative regulators of neovascularization (Good et al., 1990; O'Reilly et al., 1994; Parangi et al., 1996; Rastinejad et al., 1989). Several other endogenous inhibitors of angiogenesis have been identified, although not all are associated with the presence of a tumor. These include, platelet factor 4 (Gupta et al., 1995; Maione et al., 1990), interferon-alpha, interferon-inducible protein 10 (Angiolillo et al., 1995; Strieter et al., 1995), which is induced by interleukin-12 and/or interferon-gamma (Voest et al., 1995), gro-beta (Cao et al., 1995), and the 16 kDa N-terminal fragment of prolactin (Clapp et al., 1993). The only known angiogenesis inhibitor which specifically inhibits endothelial cell proliferation is angiostatin (O'Reilly et al. 1994).
Angiostatin is an approximately 38 kiloDalton (kDa) specific inhibitor of endothelial cell proliferation. Angiostatin is an internal fragment of plasminogen containing at least three of the five kringles of plasminogen Angiostatin has been shown to reduce tumor weight and to inhibit metastasis in certain tumor models. (O'Reilly et al., 1994). As it is used hereinafter, the term "angiostatin" refers to angiostatin as described above; peptide fragments of angiostatin that have endothelial cell proliferation inhibiting activity; and analogs of angiostatin that have substantial sequence homology (as defined herein) to the amino acid sequence of angiostatin, which have endothelial cell proliferation inhibiting activity.